Poeciliid fishes have heterogametic sex determination but with sex chromosomes that are considered "primitive" because they bear several quantitative trait loci and show little morphological variation. Thus, they are thought to represent an intermediate stage in the evolution of the highly specialized sex chromosomes seen in mammals. The linkage of quantitative traits that are beneficial to one sex but disadvantageous to the other may be a driving force in the specialization of sex chromosomes. The sailfin molly (Poecilia latipinna) serves as a model for studies of the maintenance of genetic polymorphism in natural populations. A single gene on the Y chromosome of this species has been documented to yield major phenotypic effects on male size and age at maturity with documented fitness consequences. Sex-linked genes are responsible for a variety of disorders in humans and our study system can serve as a model for the action of selection on Y-linked traits and the use of molecular genetic markers in such a study. The goals of the proposed research are to identify markers on the Y chromosome of this species, screen for markers linked to the locus determining male size, and survey three populations for repeatability of all markers identified. The research proposed will provide training for students in evolutionary biology of sex chromosomes and in the use of quantitative and molecular genetic techniques. The research will provide information needed to address organization of Y chromosomes and patterns of selection acting on a Y-linked trait in a model study system.